Silicon carbide (SiC) is a thermally and chemically stable compound. SiC is excellent in bandgap, breakdown voltage, electron saturation velocity and thermal conductivity as compared with silicon (Si). Therefore, SiC is expected to be used as a next-generation semiconductor material.
SiC is known as a material that occurs in many different crystal structures. The crystal structures of SiC include hexagonal polytypes 6H and 4H, a cubic polytype 3C and others. A SiC single crystal having a crystal structure of 4H (which will hereinafter be referred to as a 4H—SiC single crystal) has a wide band gap as compared with SiC single crystals having other crystal structures. Therefore, a 4H—SiC single crystal is attracting attention as a material for next-generation power devices.
As a way of producing a SiC single crystal, a solution growth technique is known. According to the solution growth technique, a crystal growth surface of a seed crystal, which is a SiC single crystal, is caused to contact a Si—C solution. The portion of the Si—C solution in vicinity to the seed crystal is put into a supercooled state, whereby a SiC single crystal grows on the seed crystal.
Regarding growth of a 4H—SiC single crystal, a spiral growth process is known as a way of causing the growing crystal to inherit the polytype of the seed crystal. In the spiral growth process, a crystal is grown while information of a stacking sequence is transmitted to the crystal growth surface by use of screw dislocations existing in the seed crystal.
During bulk growth of a 4H—SiC single crystal, it often occurs that polytypes other than the polytype 4H are mixed in the growing crystal. This results in the production of a SiC crystal partly including polycrystals (defects). Thus, stable growth of a 4H—SiC single crystal is difficult.
There are two approaches to stable growth of a 4H—SiC single crystal. One is causing the growing single crystal to inherit the polytype of the seed crystal (that is, the polytype 4H), and the other is performing nucleation of 4H—SiC. In the latter approach, it is difficult to control the polytype. In the former approach, the polytype (4H) of the seed crystal is not always inherited to the growing single crystal successfully. A failure in inheriting the polytype of the seed crystal to the growing crystal results in mixing of different polytypes (6H and/or other polytypes) other than 4H—SiC in the growing crystal, which makes it difficult to grow a 4H—SiC bulk single crystal.
Japanese Patent Application Publication No. 2009-91222 (Patent Literature 1), International Patent Application Publication No. 2013/065204 (Patent Literature 2) and Japanese Patent Application Publication No. 2014-122133 (Patent Literature 3) disclose SiC single crystal production methods that inhibit defects in SiC single crystals.
In the production method disclosed in Patent Literature 1, a SiC seed crystal having a crystal growth surface inclined from the {0001} plane is immersed in a Si—C solution, and a SiC single crystal is grown on the seed crystal. According to Patent Literature 1, this inhibits three-dimensional growth of the SiC single crystal and promotes stable and high-grade two-dimensional growth.
In the production method disclosed in Patent Literature 2, the degree of C supersaturation in the SiC solution in the region in vicinity to the crystal growth surface of the SiC seed crystal is controlled. Specifically, a high C supersaturation period and a low C supersaturation period are repeated alternately. According to Patent Literature 2, in this way, a SiC single crystal having an even crystal growth surface can be produced at a high growth rate.
In the production method disclosed in Patent Literature 3, in the middle of growth of a SiC single crystal, the crystal growth surface is separated from the Si—C solution to stop the crystal growth. After the stop of the crystal growth, the temperature of the Si—C solution is raised. After the temperature rise of the Si—C solution, which leads to maintenance of a constant temperature of the Si—C solution, the discontinued growth of the SiC single crystal is restarted. According to Patent Literature 3, the C concentration in the Si—C solution is regulated in this way, which leads to inhibition of changes and transitions of the polytype.